1. Field of the Invention
The present invention relates to a disk-type storage medium and, more particularly, to a driving device generally referred to as disk drive, suitable for driving magnetic disks which are used as storage media in a magnetic-disk-type storage apparatus.
2. Description of the Related Arts
A typical known disk drive used in a magnetic-disk-type storage device employs a hub and a clamp which cooperate with each other in clamping a stack of disks therebetween. More specifically, a plurality of disks and spacers are stacked alternately on the hub, and the outer peripheral region of the clamp presses the disks and spacers in a stacking direction thereby fixing the disks and spacers to the hub.
In recent years, there is a trend towards reduced size and enhanced storage capacity of magnetic-disk-type storage apparatus. In fact, 5-inch disks which were the dominant storage apparatus have already been replaced by 3.5-inch disks and, nowadays, smaller and thinner disks, e.g., 2.5-inch disks, are becoming popular. Meanwhile, storage capacity of storage apparatuses employing, for example, 3.5-inch disks has been increased from 200 megabytes to 400 megabytes and further to 1 gigabytes. Thus, disk drives have to be designed and constructed in conformity with the above-mentioned trend towards reduction in the size of apparatus and enhancement of the storage capacity. When enhancement of storage capacity is achieved by increasing the number of the disks, the disk drive is required to have a reduced thickness to reduce the overall height of the apparatus.
As stated before, a conventional magnetic disk drive has a hub and a clamp which cooperate with each other in clamping therebetween a stack of disks and spacers arranged alternately, the clamp at its outer peripheral region pressing the stack in the stacking direction thereby fixing the disks and spacers to the hub. In this known structure, the disks tend to be warped or distorted in the radial direction because of deformation of the hub caused by the pressing force exerted by the clamp, even though the force of the clamp is applied uniformly. This problem is serious, particularly in small-sized apparatuses which employ hubs of smaller rigidity, as well as spindle motors of smaller size.
FIG. 11 illustrates an example of a known disk drive which incorporates an in-hub type spindle motor. The spindle motor has a hub A which is held by bearings D, E secured to a shaft C on a bracket B. A rotor hub F fits on the hub A and the shaft C. A stator G, which is provided at its portion inside the hub with a coil, is secured to the shaft C. A magnet rotor H is attached to the inner surface of the hub. Disks I-K and spacers L -M are alternately stacked and fitted on the hub A, and are pressed towards and against a flange O of the hub by means of a clamp N which is screwed to the hub A.
The clamp N screwed to the hub A produces a force which acts, as shown in FIG. 12, in such a manner as to radially outwardly spread the flange O, causing elastic deformation of the hub A. Consequently, the disks I-K and spacers L-M are displaced, with the result that some or whole of the disks are warped depending on factors such as the thickness of the disk, number of spacers and rigidity of the hub. For instance, when the hub has a reduced mechanical strength due to requirement for reduction in the size of the apparatus, the point or area of contact between the disk I and the spacer M is located radially outside the point or area of contact between the disk I and the hub A. As a consequence, a bending moment M.sub.1 is generated to act about a fulcrum presented by the point or area of contact between the disk I and the spacer M, due to restoration force exerted by the hub, with the result that the disk I is distorted so as to convex towards the flange O or the bracket B. Alternately, the contact between the disk K and the clamp N takes place at a point or over an area which is on the radially inner side of the point or area of contact between the disk K and the spacer L, so that a bending moment M.sub.2 is generated about a fulcrum presented by the point or area of contact between the disk K and the spacer L, due to restoration force of the hub. In such a case, the disk K is warped to convex towards the clamp N.
Increase in the storage capacity of the apparatus requires not only increased density of recording of data in the disk but also enhanced resolution of recording. To this end, it is necessary to reduce the amount of lift of the magnetic head. Since the data area exists on a radially inner part of the disk, the warp of the disk at the data area is more critical in small-sized disks than in large-size disks. Consequently, a significant difference is caused in the amount of lift between the obverse side and reverse side of the disk, which in turn causes a large difference in read-out voltage, thus impairing read-out precision.
FIG. 13 illustrates a relationship between the amount of lift of a head associated with the disk adjacent to the hub and the amount of lift of a center rail portion which constitutes a signal pick-up portion. Referring to this figure, a head facing the concave side of the warped disk 1 has a slider P which is provided with floating surfaces P.sub.1 and P.sub.2 and a center rail portion P.sub.3 which constitutes the signal pick-up portion. A head facing the convex side of the warped disk 1 has a slider Q which is provided with floating surfaces Q.sub.1 and Q.sub.2 and a center rail portion Q.sub.3 which constitutes the signal pick-up portion. When the disk 1 has been warped as illustrated, the amount hg1 of lift of the center rail portion P.sub.3 tends to increase, even when the amounts h.sub.11 and h.sub.21 of lifts of the floating surfaces P.sub.1 and P.sub.2 are maintained stably. Conversely, in case of the head facing the convex side of the disk 1, the amount hg.sub.2 of lift of the center rail portion Q.sub.3 tends to decrease, even when the amounts h.sub.12 and h.sub.22 of lifts of the floating surfaces Q.sub.1 and Q.sub.2 are maintained stably. Consequently, a large difference is developed in the read-out voltage between the head facing the concave side of the disk 1 and the head facing the convex side of the same. In particular, read-out precision is liable to be impaired on the slider P facing the concave side of the disk due to reduced read-out voltage, because the amount hg.sub.2 of lift of the center rail portion exceeds the design value.
Japanese Patent Unexamined Publication No. 2-58781 discloses a magnetic-disk-type storage apparatus in which spacers between disks of a stack are made of an elastic material so as to reduce distortion of disks which are caused by the pressure applied by the clamp. Practically, however, it is impossible to obtain spacers of an elastic material with a high degree of dimensional precision. The dimensions of the spacer of such elastic material largely changes according to factors such as temperature and clamping pressure, so that the dimensional precision of the spacers varies according to individual apparatus, resulting in variation in the lift amounts of the heads. It is therefore difficult to produce a magnetic-disk-type storage apparatus having a high read-out precision using such spacers.